Clarifiers using dissolved air flotation (DAF) to remove impurities from water or wastewater are known. As used herein, the term clarifier includes any device that removes impurities from a liquid including but not limited to roughing filters or polishing filters.
A typical prior art DAF basin is illustrated in FIGS. 1 and 2. A flocculator A is employed with the clarifier B. The flocculator A may contain one or more flocculation zones (two flocculation zones 1 are shown in FIG. 2). Typically, a coagulant is added to the liquid to be filtered prior to entry into the flocculator A. The coagulant acts to destabilize the colloidal particles which facilitates their agglomeration in the flocculator A. Once the liquid passes upwardly through the flocculator A it travels downwardly though the inlet chamber or zone C. The liquid subsequently passes into the reaction zone D. A saturated water distributor 2 is located adjacent the bottom of the reaction zone D. As shown in FIG. 2, the distributor 2 is connected to a saturator 3 which is connected to an air compressor 5. The saturator provides water saturated with air to the distributor 2. A plurality of DAF nozzles 4 extend from distributor 2.
The saturated water flowing through the distributor 2 is depressurized as it passes through the DAF nozzles 4 thereby creating a multitude of micro-bubbles that mix with the liquid to be clarified in the reaction zone D. The flocculated particulate in the liquid to be clarified agglomerates with the micro-bubbles and forms a thick layer of sludge on the surface of the clarification zone E. The clarified liquid subsequently passes into the effluent channel F. When the layer of sludge (or float) sufficiently builds up, it can be removed by a variety of mechanical and hydraulic means. In this example, a desludge valve opens dropping the level in the desludge chamber 9 thereby allowing the sludge layer or float to flow over weir 8 into desludge chamber 9.
Typically, a recycle pump 10 is connected to the effluent channel F via recycle line 11 to recycle a portion of the clarified liquid, i.e., a portion of the clarified liquid is fed back to the saturator 3. A significant problem has been encountered by a number of previously known DAF clarifiers when the flow rate is increased above 8 to 10 gpm/sq.ft. Specifically, micro-bubbles become entrained in the clarified liquid traveling through the effluent channel F. As shown in FIG. 1, this occurs because the flow conditions in the clarification zone E (see arrows 12) are such that the lower edge 14 (represented by solid curved line in clarification zone K in FIG. 1) of the “white water” (i.e., a mixture of micro-bubbles and liquid to be clarified) extends below the inlet 16 of the effluent channel F during higher filtration rates allowing the micro-bubbles to readily pass into the effluent channel F. One previously known solution to this problem is to significantly increase the height of the clarifier in order to raise the lower edge of the white water to above the inlet 16 of the effluent channel F. However, this is extremely undesirable because it significantly increases the size and cost of the clarifier.